As
expected, the dominant constituents of the atmosphere are
hydrogen and helium. But the amount of helium -- about 15
percent -- was considerably less than the 40 percent that
had been suggested by some Earth-based studies. Methane,
acetylene and other hydrocarbons exist in much smaller quantities.
Methane in the upper atmosphere absorbs red light, giving
Uranus its blue-green color.

Voyager
images showed that the atmosphere is arranged into clouds
running at constant latitudes, similar to the orientation
to the more vivid latitudinal bands seen on Jupiter and
Saturn. Winds at mid-latitudes on Uranus blow in the same
direction as the planet rotates, just as on Earth, Jupiter
and Saturn. These winds blow at velocities of 40 to 160
meters per second (90 to 360 miles per hour); on Earth,
jet streams in the atmosphere blow at about 50 meters per
second (110 mph). Radio science experiments found winds
of about 100 meters per second blowing in the opposite direction
at the equator.

A
high layer of haze -- photochemical smog -- was detected
around the sunlit pole.

The
sunlit hemisphere also was found to radiate large amounts
of ultraviolet light, a phenomenon that Voyager scientists
have dubbed "dayglow."

The
average temperature on Uranus is about 60 Kelvin (- 350
degrees Fahrenheit). The minimum near the tropopause is
52 K (-366 F) at the 0.1-bar pressure level. (The tropopause
is the boundary between the stratosphere and the troposphere,
the lowest level of atmosphere, comparable to the region
on Earth where life abounds. One bar is the average pressure
at sea level on Earth.)

Surprisingly,
the illuminated and dark poles, and most of the planet,
show nearly the same temperature below the tropopause. Voyager
instruments did detect a somewhat colder band between 15
and 40 degrees latitude, where temperatures are about 2
to 3 K lower. The temperatures rise with increasing altitude,
reaching 150 K (-190 F) in the rarified upper atmosphere.
Below this level, temperatures increase steadily to thousands
of degrees in the interior.